Composition and method for the dissolution of gypsum

ABSTRACT

THE PRESENT INVENTION RELATES TO A COMPOSITION AND METHOD FOR THE DISSOLUTION OF GYPSUM WHEREIN SAID GYPSUM IS CONTACTED WITH A SOLUTION WHICH CONVERTS IT INTO A DISPERSIBLE SLUDGE.

United States Int. Cl. C23g N02 US. Cl. 252-82 11 Claims ABSTRACT OF THEDISCLOSURE The present invention relates to a composition and method forthe dissolution of gypsum wherein said gypsum is contacted with asolution which converts it into a dispersible sludge.

The present invention relates generally to a composition and method foruse in converting gypsum scales or deposits to a dispersible sludge.

Undesirable deposits of gypsum (CaSO .2H O) occur frequently inindustrial boilers, heat exchangers, water systems and in subterraneanformations used as a source of water and hydrocarbons. The depositsgenerally cause clogging or reduced flow in the water system or earthenformations where they are found.

Gypsum deposits are found in producing wells, injection wells, and wastedisposal. Deposits of gypsum can significantly reduce the flow of oiland water into the wellbore by plugging-off perforation in the tubingand/ or casing. In addition, downhole pumps can become stuck as a resultof gypsum scale deposition. Gypsum deposit can form on the surface oftubular goods in the well in such a manner as to severely impedeproduction of fluids.

Two general types of more or less effective compositions have been usedin the past to remove gypsum deposits. Carbonate solutions have beenused to convert gypsum to calcium carbonate which is then removed withan acid. Besides requiring the use of an acid to dissolve the calciumcarbonate, the carbonate compositions show reduced effectiveness ondense, laminated gypsum scales, requiring repeated treatments with thecarbonate composition and repeated washings with an acid. Acids aregenerally undesirable in that they may be corrosive to the system orformation from which the gypsum deposit is sought to be removed.

Sodium and potassium hydroxides have also been used with someeffectiveness to remove gypsum deposits. The hydroxide converts thegypsum to calcium hydroxide which is soluble in acid. In some cases thecalcium hydroxide will be in the form of a sludge which may be pumped orWashed away from the system or formation, while in other instances, acidmust be used to remove the reaction precipitate from the well. Althougha fair degree of success has been obtained with the hydroxides, in somecases the hydroxides tend to decrease effectiveness with repeatedtreatments, partially because of a buildup of the calcium hydroxidesludge within the system and because of the hydroxide coating formed onthe surface of the gypsum deposit. The hydroxides also sometimes requirethe undesirable use of an acid to wash away the reaction precipitate.

The present invention relates to a solution for the couatent "iceversion of gypsum deposits to a dispersible sludge comprising the saltof an alkali metal and a monocarboxylic acid and the method of its use.

The present invention provides a composition and method for convertinggypsum deposits to a dispersible sludge, eliminating the necessity of anacid treatment to disperse the treated gypsum deposits and greatlyimproving the efficiency in pounds of gypsum removed per gallon ofsolution used over the prior art.

The said solution, when in contact with a gypsum deposit, will reactwith the gypsum in such a Way as to form a precipitate, klauzite, whichwill slough away from the surface of the gypsum, thereby increasing thepenetration of the solution. When the solution contacts the gypsumdeposit, the following reaction is thought to occur:

Potassium acetate, the most preferred chemical composition, is used inthe above sample reaction.

Because the klauzite, K Ca(SO .I-I O, does not adhere tightly to thegypsum deposit and is readily dispersible in water, an acid stage is notnecessary after treating with the solution of the invention.

It is an advantage of the solution of this invention that no subsequentacid treatment is needed, the precipitate being a dispersible sludge.However, should an acid treatment following the use of the solution ofthe invention be desirable for cleaning or some other purpose, theklauzite is soluble in acid to the extent of 1.2 pounds per gallon in15% HQ. Klauzite is soluble in water to the extent of 0.0207 pound pergallon.

The klauzite which forms as the solution of this invention reacts withthe gypsum deposits does not adhere tightly to the said deposit, butsloughs away, overcoming the disadvantage of gypsum removal solutionswhich form a reaction product that tightly adheres to the said deposit,thereby stopping the reaction and requiring an acid to remove saidreaction product.

Increased efiiciency of gypsum removal is another advantage of thesolution of this invention. At 72 F. a gypsum solvent having aconcentration of 30% of the solution of the present invention willconvert to a dispersible sludge 1.2 pounds per gallon of gypsum, whilesodium hydroxide followed by an acid will remove only .50 pound pergallon of the same type gypsum, and sodium carbonate followed by an acidwill remove .35 pound of gypsum per gallon of solution during a 24-hourperiod. The solution of the invention also shows a marked increase inefficiency over the hydroxide gypsum removal solutions, as well as overthe carbonate gypsum removal solutions on various samples of gypsum withthe increase in efficiency widening with time. The solution of thisinvention shows an efficiency improvement of up to five times theability to remove gypsum over solvents commonly used in the art.

The solution of this invention will operate in a pH range of from aboutpH 6 to about pH 14 to convert gypsum deposits to a dispersible sludge;however, the preferred pH range is found to be 9 to 11 and mostpreferred pH is 10. Although the gypsum removal solution of thisinvention works well in a basic environment, a

difference of about .44 pound per gallon is noticed in the ability ofsaid solution to remove gypsum deposits at a pH of about 6 and at a pHof about 12, with the maximum removal ability found at about pH 10 inmost cases.

The concentrations of the monocarboxylic acid salt, potassium acetate,varies from 0.5% by weight to 75% by weight depending on the amount ofgypsum to be removed, the system from which it is to be removed and theanticipated agitation of the solution of the present chain organicsolvent such as kerosene or diesel to aid in removing paratrins andcongealed oil which may be co-deposited with the gypsum deposit.

The invention will be further illustrated by the following examples. Itshould be understood, however, that although these examples maydescribe, in particular detail, some of the more specific features ofthe invention, they are given primarily for the purpose of illustration;and the invention in its broader aspects is not to be coninvention inthe presence of the gypsum deposit. Clean- 1O Strued as limited b hfollowin examples.

ing a thin layer of gypsum deposit from a large volume boiler mightrequire a concentration of potassium acetate TEST PROCEDURE from about1.0% to about 5.0% while cleaning gypsum geposits. from Subterraneanformation orpil Well q The gypsum solubility tests described below areconmg eqmpmept could require a concentranon of potasilum ducted byplacing a Weighed gypsum block in contact with acetate tagging fromabout 10% to Higher 100 cubic centimeters of the solution described inthe inconcentratrons of the elements of this invention and 11'1- ventionfor eriods of 24 48 and 72 hours at F After creased temperatures appearto increase its rate of opsoaking forp24 hours gypsum cube is removedfrom era 10D.

The preferred elements of the composition of this inigsve igllglbggslslzglewgg223$: girfievtviasvlvliitnhg aizftttonelaattgd ventron1s the potassium salt of acetic acid in an aqueous d d d d solution.Other compositions of alkali metals and monorymg.wlt acetone an rewelgmg 18 use to etermme carboxylic acids may be used effectively in Water,e.g., the Welght loss of the test cube at 48 and 72 hours the ammonium,sodium and potassium salts of formic specnvely' E AMPL and propionicacids and including the ammonium and 25 X E 1 sodium salts of aceticacids.

Gypsum deposits are removed by contacting the said Table I shows hamount of gypsum removed m grants deposits with the solution of theinvention for a time Per 100 cublc Femlmeters Pounds Per sufficient toconvert said deposits to a pumpable Sludge ous concentrations ofpotassium acetate whrlevarying the and by removing the dispersiblesludge after the said so- PH and solvatlon A Solunon contammg 32'73%lution has converted the said deposits. The details of the process willvary depending on the system from which the gypsum deposits are to beremoved.

Although the solution of the present invention is most often used inaqueous solutions, it is advantageous to form a dispersion or emulsionof the monocarboxylic salt with an aromatic solvent such as xylene or astraight- TABLE L-DISSOLUTION OF GYPSUMVIT IA potassuim acetate salt(weight to weight) with the pH of 8 will remove approximately 0.53, 0.85and 0.99 pound of gypsum per gallon solution in 24, 48 and 72 hoursrespectively at ambient temperatures under static conditions. A solutioncontaining 49.02% potassium acetate salt (weight to Weight) at a pH of 8completely disintegrated the test block.

VARIOUS CONCENTRATIONS OF POTASSIUM ACETATE ILE VARYING pH 0380421120dissolved Percent acid silt Gms. aiter- Lbs/gal. aiter- Y Acid saltweight pH 24 hr 48 hr. 72 hr. 24 hr. 48 hr. 72 hr. Comments 6. 0 2. 0 2.8 16 Potassium acetate".-. 16. 34 g: 3 i? jg 12.0 0.3 1.4 .025 6.0 2.03.9 .16 Potassium acetstc..... 16.34 I 12.0 1.3 2.4 .10 6. 0 6. 6 9. 5.64 Potassium acetate"... 32. 78 75 91 12. 0 2. 7 9. 0 .22 75 1. 0 Blockwas soit and crumbly after 48 hrs.

6.0 6.4 9.1 .45 .75 .93 Potassium acetate... 32. 78 8:3 5:: 1 2% 2 2g:82

12. O 4. 2 10. 1 35 84 1.04 Block was soft and crumbly after 48 hrs.

6. 0 34. 4 34. 4 2. 87 2. 87 Block completely crumbled in 24 hrs.Potassium acetate 32. 78 8. 0 36.0 36.0 3.00 Do. 10. 0 38. l. 38. 1 3.10 D0.

6. 0 9. 9 15.6 1. 3 1. 46 Block completely crumbled in 72 hrs. Potassiumacetate 4902 Blococlompletely crumbled in 48 hrs.

'8 iii ii': i' 2' Bi 1: hi d1 7211 8. 44 5 0e crum e n rs. Potassiumacetate 02 13. g 24.6 Blocg completely crumbled in 24 hrs.

1 Dissolution, as referred to here, indicates the loss in wt. of a blockof gypsum, as result of reaction with chemical solutions. I Gypsumobtained from difierent sample from the Southard, Oklahoma quarry.

I 3,706,669 s 6 EXAMPLE 2 Table II shows that the potassium salts ofpropionic acid will also convert gypsum to a pumpable sludge.

TABLE 11.-DISSOLUTION OF GYPSUM IN POTASSIUM PROPIONATE OaSOfiHzOdissolved Percent Gms./100 cc. acid sglt after- Lbs/gal. a!ter Y Acidsalt weight pH 24 hr. 48 hr. 72 hr. 24 hr. 48 hr. 72 hr. CommentsPotassium propionate. 30. 28 8. 2. 0 6. 0 31. 9 16 0. 2. 5 Blockcompletely crumbled in 72 hrs.

D0 30. 28 10. 0 2. 5 13. 0 38. 9 1. 0S 3. 2 Do.

EXAMPLE 3 15 Table III shows a comparison of sodium,'ammonium andpotassium salts of acetic acid. This data indicates that potassiumacetate is the preferred.

TABLE III-COMPARISON OF DISSOLUTION OF GYPSUM IN POTASSIUM ACETATESOLUTIONS WITH SODIUM ACETATE AND AMMONIUM ACETATE SOLUTIONS AT VARIOUSpH SOLUTIONS CaSO4-2HQO dissolved Percent acid Gals/100 cc. Lbs/gal.sglt aiteratter-- y Acid salt weight pH 24 hr. 48 hr 24 hr. 48 hr.Comments Sodium acetate reactlon precipitate tended to adhere to Sodiumacetate 26.24 8.0 .8 1.0 .06

i 10.0 2. 2 2. a .18 24 i the gyp block Ammonium acetate 25 66 Ammoniumacetate reaction precipitate tended to adhere 10.0 0.8 1. 4 .11 .11 gypblock- 6.0 9.5 11.5 .79 .95 Potassium acetate reaction precipitatesloughed off the Potassium acetate 32. 78 8.0 10.6 12.6 .88 1.05 surface01 gyp block more readily than other acetate 10.0 9. 0 11.0 .95 .91salts.

EXAMPLE 4 Table IV shows the effect of temperature on the convertingproperties of potassium acetate. Increasing the temperature from F. to150 F. appeared to increase 40 the rate of conversion of gypsum by thepotassium acetate solution.

TABLE IV.-DISSOLUTION OF GYPSUM IN POTASSIUM ACETATE SOLUTIONS AT 150 F.

C3SO4'2H30 dissolved Percent acid Gms./100 cc. Lbs/gal. salt afteraiteryAcid salt weight pH 24 hr. 48 hr. 24 hr. 48 hr. Comments Potassiumacetate 16. 34 Potassium acetate 8. 0 23.9 23. 9 1.99 1. Blockcompletely crumbled in less than 24 hours.

8. 0 9.0 23. 4 .75 1. Block completely crumbled in 48 hrs. Potassiumacetate 8. 0 28. 8 Block completely crumbled in less than 24 hrs.

EXAMPLE 5 Table V gives a comparison of potassium salts of formic acid,acetic acid, propionic acid and butyric acid.

TABLE V.COMPARISON OF DISSOLUTION OF GYPSUM IN POTASSIUM ACETATE SOLU-TIONS WITH OTHER POTASSIUM SALTS OF MONO-CARBOXYLIO ACIDS CaSO4-2HgOdissolved Gms./ cc. Lbs/gal. afterafter- Percent Acid salt Salt acid pH24 hr. 48 hr. 24 hr. 48 hr. Comments 6. 0 0.0 0. 2 0.0 0.010 Potassiumtor-mate 36. 54 20 8. 0 0.0 0. 3 0.0 0. 025 10.0 0.0 0. 1 0.0 O 008 6. 09.5 11. 5 .79 Potassium acetate 36. 54 20 8.0 10.6 12.6 .88 1.05 10.09.0 11.0 75 .91

6. 0 Not determined. Potassium propionate..- 30.28 20 8.0 2.0 6.0 .16.50

10.0 2.5 13. 0 .20 1. 08 Block completely crumbled in 72 rs. 6. 0 1 1008 008 Potassium butyrate- 28. 64 20 8.0 .1 .4 008 .032 10.1 1 1 008008 7 EXAMPLE 6 Table VI shows the improvement of the present inventionover compositions presently in general use for the dissolution anddispersion of gypsum deposits.

solution is in the range of about 9 to about 11.

TABLE VI.-COMPARISON OF VARIOUS TYPES OF CHEMICALS USED FOR GYPSUMREMOVAL [Vol.=100 ml.; Solvation period: 24 and 48 hrs.;(;oncentration=wt. percent;

temperature 72 F.

Gypsum removed (lbs/gal.)

24 hours 48 hours Scale Scale Scale Scale Scale Scale #1 #2 #3 #1 #2 #3Solvent types:

Inorganic converters: l

12% NHHCOS'NQCOS- 0.06 0.0 0. 03 0.06 0.1 0.30 KOH 0.00 0.0 0.0 0.000.00 0.0 0.23 O. 17 0. 17 0. 60 0. 4 0. 17

0.35 0.37 0. 40 0. 6 0. 74 0. 80 0.33 0.0 0.50 0. 5 0.30 0.65 0. 50 0. 60. 52 0. 9 0. 80 0. 76 Solvent types:

Organic converters: 1

potassium acetate 1. 2 0. 90 0.7 3.0 1.7 1. 50 30% Na citrate 0. 0. 560. 5 0. 88 1.0 0.78 30% K glycolate--- 0. 36 0. 0. 5 0. 45 0. 66 0. 94Solvents: 1

30% NarEDTA. 0.23 0. 31 0 25 0. 43 0. 45 0.36 30% NazNTA 0.01 0.04 0 010.01 0.04 0.01

1 No acidizing after each solvation period.

Acidized for 2 hours in 15% H01 atter each solvation period.

No'rE: Scale #1=Quarry deposit, Southard, Oklahoma; Scale #2=Largecrystal from Jordan Field, Andrews 00.; Scale #3=Small needle likecrystals from Magutex Field, An-

drews Co.

EXAMPLE 7 Table VII shows a typical use for the present invention in thearea of oil production. Experiments were conducted in six (6) west Texasoil wells to determine the utility and efficicncy of the presentinvention for removing gypsum deposits. The results are shown in thetable below in barrels per day and show an improvement in oil or waterproduction.

TABLE VIL-WELL TREATMENT HISTORIES Production Before After Volume AreaTreating solution Field Formation (gaL) Oil Water 011 Water CedarLake..- San Andres- 1, 000 80 6 238 7 West Texas- Potassium acetate d 1,500 93 8 122 14 .do ..do 1, 500 60 17 102 30 Geo. Smith- Clear Fork-. 2,000 36 36 67 38 Slaughter- San Andres. 1, 500 11 3 6 Although it is nota requirement of the invention, a surfactant may be added to thesolution to improve wettability of the gypsum deposits being treated.

The above disclosure is illustrative and descriptive of the present andsuitable variations may be made without 55 departing from the spiritscope of the invention.

What is claimed is:

1. A process for the dissolution of gypsum comprising contacting saidgypsum with an aqueous solution of a 10. The process of claim 9 whereinthe pH of said solution is in the range of about 9 to about 11.

11. The process of claim 10 wherein said solution is dispersed with asolvent selected from aromatic and straight-chain organic sol-ventsprior to said contacting.

References Cited UNITED STATES PATENTS salt of a monocarboxylic acid andmaintaining said contact for a time sufiicient to convert said gypsum toa dis- 2910436 10/1959 Fan 252' 8'55 X persible sludge, wherein saidsalt is selected from alkali 3,447,963 6/1969 Callahan 252-30 X metalsalts and ammonium salts of said monocarboxylic 3 5,477 8/1961 Florence252-82 X acid and mixtures thereof, and further wherein the pH 3,170,8152/1965 White 252-855 X of said solution is in the range of about 6 toabout 14. 5 3,240,627 3 /1966 whi 252 55 X 2. The process of claim 1wherein said monocarboxylic 3,360,399 12/1967 Knox 134 3 acid isselected from the group consisting of formic acid, 3,527,609 9/1970Vinso 134 3 acetic acid, propionic acid and butyric acid.

3. The process of claim 1 wherein said alkali metal is one of potassiumand sodium.

4. The process of claim 1 wherein the concentration of said salt in saidsolution is in the range of about 0.5 to about percent by weight of saidsolution.

5. The process of claim 1 wherein said alkali metal is potassium andsaid monocarboxylic acid is acetic acid.

ROBERT F. BURNETT, Primary Examiner M. E. McCAMISH, Assistant ExaminerUS. Cl. X.R.

